Re: [PATCH] KVM: x86: fix root cause for missed hardware breakpoints

[Nadav Amit <nadav.amit@gmail.com>]

Xiao Guangrong <guangrong.xiao@linux.intel.com> wrote:

> 
> 
> On 02/26/2016 07:46 PM, Paolo Bonzini wrote:
>> Commit 172b2386ed16 ("KVM: x86: fix missed hardware breakpoints",
>> 2016-02-10) worked around a case where the debug registers are not loaded
>> correctly on preemption and on the first entry to KVM_RUN.
>> 
>> However, Xiao Guangrong pointed out that the root cause must be that
>> KVM_DEBUGREG_BP_ENABLED is not being set correctly.  This can indeed
>> happen due to the lazy debug exit mechanism, which does not call
>> kvm_update_dr7.  Fix it by replacing the existing loop (more or less
>> equivalent to kvm_update_dr0123) with calls to all the kvm_update_dr*
>> functions.
>> 
>> The original patch is good enough for stable releases before 4.1, since
>> it does not have any dependencies such as commit ae561edeb421 ("KVM:
>> x86: DR0-DR3 are not clear on reset", 2015-04-02).
>> 
>> Cc: stable@vger.kernel.org   # 4.1+
>> Fixes: 172b2386ed16a9143d9a456aae5ec87275c61489
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>>  arch/x86/kvm/x86.c | 9 ++++-----
>>  1 file changed, 4 insertions(+), 5 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index f4891f2ece23..eaf6ee8c28b8 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -2752,7 +2752,6 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>>  	}
>> 
>>  	kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
>> -	vcpu->arch.switch_db_regs |= KVM_DEBUGREG_RELOAD;
>>  }
>> 
>>  void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>> @@ -6619,12 +6618,12 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>  	 * KVM_DEBUGREG_WONT_EXIT again.
>>  	 */
>>  	if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)) {
>> -		int i;
>> -
>>  		WARN_ON(vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP);
>>  		kvm_x86_ops->sync_dirty_debug_regs(vcpu);
>> -		for (i = 0; i < KVM_NR_DB_REGS; i++)
>> -			vcpu->arch.eff_db[i] = vcpu->arch.db[i];
>> +		kvm_update_dr0123(vcpu);
>> +		kvm_update_dr6(vcpu);
>> +		kvm_update_dr7(vcpu);
>> +		vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
>>  	}
> 
> It is good to me now. It also can fix the case that the debug registers are
> changed by the debug-process(gdb/perf) in the context of the vcpu as i
> described in another thread.

Seems correct. The code may be easier to follow if the following line:

vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_WONT_EXIT

would have been moved out of svm_sync_dirty_debug_regs and
vmx_sync_dirty_debug_regs into the flow of vcpu_enter_guest (above).

Nadav